Some types of off-gases discharged from a producing line of semiconductors contain impurities, such as gases and particles, which are sometimes harmful to human beings and cause erosion to the equipment of the producing line. Therefore it is necessary to remove the above mentioned impurities from the off-gases by means of a scavenging process and it is also necessary to measure residual impurities of the off-gases after the scavenging process.
For the scavenging and the measuring of the off-gases, the following scavenging system is used in a producing line of semiconductors. The scavenging system scavenges impurities from the air of the clean room, in which the semiconductors are produced, by discharging the air into the scavenging liquid, such as the demineralized water in the scavenging unit, so as to dissolve the impurities into the scavenging liquid. Thus, scavenged impurities in the scavenging liquid are analyzed by the means of microanalysis such as ion chromatography.
In the impurity scavenging system described above, the density of the impurities of the scavenging liquid in the scavenging unit increases during a period of continuous scavenging. Therefore the scavenging unit, the condenser and the pipe lines can be contaminated by the scavenging liquid which contains a high density of the impurities. Some errors in the measurement of the density of the impurities can be caused by this contamination. It requires much effort for cleaning the scavenging system when it is contaminated by impurities in order to avoid the errors in measuring described above.
The scavenging unit should be filled with a specified quantity of new scavenging liquid after the cleaning of the scavenging unit, the condenser and the pipe lines before the starting of the scavenging system. Therefore it is necessary to measure the quantity of the new scavenging liquid by means of an additional measuring means, such as a flow meter or level meter. However, a controlling system for the scavenging system becomes more complicated by the addition of this measuring means.